Gain expansion occurs when the gain of an amplifier stage increases as its input drive power increases due to the signal dependent change in the bias point of the amplifier. Thus, the magnitude of gain expansion correlates to the bias point of the amplifier stage. As the amplifier quiescent bias is reduced toward Class B operation, the increasing gain expansion introduces a trade-off between linearity and efficiency.
Current single chain, multi-mode linear heterojunction bipolar transistor (HBT) radio frequency (RF) power amplifiers (PA) suffer from large gain expansion when they are biased at lower quiescent current levels in order to reduce the amplifier's gain and power output in medium or low power modes. The excess gain expansion can be alleviated by increasing the quiescent bias current to the level that limits gain expansion and achieves sufficient linearity at the desired output power level. However, if the increased quiescent bias current leads to excess power gain in the low or medium power mode, the gain must be reduced to the specified target by switching in a series attenuator or by switching in additional feedback in the early gain stages. This approach limits the minimum level of quiescent bias current that can be achieved and requires additional circuitry to be added, such as FET switches, to achieve the additional power modes.